In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with the mass-scale fabrication of 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was abandoned and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line edge or width roughness (LER, LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was abandoned because LUAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern features. A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
As compared with the line pattern, the hole pattern is difficult to reduce the feature size. In order for the prior art method to form fine holes, an attempt is made to form fine holes by under-exposure of a positive resist film combined with a hole pattern mask. This, however, results in the exposure margin being extremely narrowed. It is then proposed to form holes of greater size, followed by thermal flow or RELACS® method to shrink the holes as developed. However, there is a problem that control accuracy becomes lower as the pattern size after development and the size after shrinkage differ greater and the quantity of shrinkage is greater. With the hole shrinking method, the hole size can be shrunk, but the pitch cannot be narrowed.
It is then proposed in Non-Patent Document 1 that a pattern of X-direction lines is formed in a positive resist film using dipole illumination, the resist pattern is cured, another resist material is coated thereon, and a pattern of Y-direction lines is formed in the other resist film using dipole illumination, leaving a lattice-like line pattern, interstices of which provide a hole pattern. Although a hole pattern can be formed at a wide margin by combining X and Y lines and using dipole illumination featuring a high contrast, it is difficult to etch vertically staged line patterns at a high dimensional accuracy. It is proposed in Non-Patent Document 2 to form a hole pattern by exposure of a negative resist film through a Levenson phase shift mask of X-direction lines combined with a Levenson phase shift mask of Y-direction lines. However, the crosslinking negative resist film has the drawback that the resolving power is low as compared with the positive resist film, because the maximum resolution of ultrafine holes is determined by the bridge margin.
A hole pattern resulting from a combination of two exposures of X- and Y-direction lines and subsequent image reversal into a negative pattern can be formed using a high-contrast line pattern of light. Thus holes having a narrow pitch and fine size can be opened as compared with the prior art. However, two exposures must be performed while exchanging the mask, leaving problems including reduced throughputs and misregistration between two exposures.
Non-Patent Document 3 reports three methods for forming hole patterns via image reversal. The three methods are: method (1) involving subjecting a positive resist composition to two double-dipole exposures of X and Y lines to form a dot pattern, depositing a SiO2 film thereon by LPCVD, and effecting O2-RIE for reversal of dots into holes; method (2) involving forming a dot pattern by the same steps as in (1), but using a resist composition designed to turn alkali-soluble and solvent-insoluble upon heating, coating a phenol-base overcoat film thereon, effecting alkaline development for image reversal to form a hole pattern; and method (3) involving double dipole exposure of a positive resist composition and organic solvent development for image reversal to form holes.
The formation of negative pattern through organic solvent development is a traditional technique. A resist composition comprising cyclized rubber is developed using an alkene such as xylene as the developer. An early chemically amplified resist composition comprising poly(t-butoxycarbonyloxystyrene) is developed with anisole as the developer to form a negative pattern.
Recently a highlight is put on the organic solvent development again. It would be desirable if a very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone exposure. To this end, a positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. An attempt to double a resolution by combining two developments, alkali development and organic solvent development is under study.
As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. One pattern forming process is described in Patent Document 1.
In the practice of organic solvent development, a negative pattern can be formed using a common resist material which functions as a positive resist material on alkaline development and a specific organic solvent as the developer. The dissolution rate of unexposed region in organic solvent developer is one or two orders of magnitude lower than the dissolution rate of exposed region in alkaline developer. This implies that if exposure light is of the same optical contrast, the negative pattern resulting from organic solvent development has a narrower margin and a lower maximum resolution than the positive pattern resulting from alkaline development.
When negative development in organic solvent is carried out using a positive resist material, the resist film formed after development contains a large amount of methacrylic acid with the protective group deprotected. Methacrylic acid has little etch resistance because it is free of cyclic structure and rich in oxygen. The negative development has the problem that the resist film after development has so weak etch resistance that dimensional variations may occur during etching transfer of the resist pattern. This is demonstrated in Non-Patent Document 3 reporting the organic solvent development process wherein the dimensional variation after etching is greater than the dimensional variation after development.
Since the organic solvent development process uses organic solvents as both developer and rinse, it is very expensive as compared with the conventional process using alkaline developer and water rinse. It is a negative resist material that can form a negative pattern through common alkaline development. However, crosslinking negative resist material suffers from low maximum resolution due to swell during development. It would be desirable to have a negative resist material which is applicable to alkaline development and exhibits high resolution.
Non-Patent Document 4 reports a dual-tone resist material having both acid generator and base generator added. The material is based on a polymer having both an acid labile group and an adhesive group as used in conventional positive resist materials. A low exposed region forms a positive pattern and a more exposure dose region becomes a negative pattern. Using the material, lines of negative pattern can be formed between lines of positive pattern through a single exposure, leading to a doubling of resolution.
Non-Patent Document 5 proposes a resist material comprising a polymer comprising methacrylate substituted with t-butyl as an acid labile group and maleic anhydride copolymerized therewith, a thermal acid generator in the form of benzylcyclopentylsulfonium trifluoromethanesulfonate, and a photobase generator. The photobase generator generates a base upon exposure to light, and the thermal acid generator generates an acid during high-temperature PEB. The negative pattern formed by alkaline development is converted, through silylation of maleic anhydride with diaminosiloxane and dry etching, into a negative pattern with a high aspect ratio.